Capacitors are a fundamental building block in electronic devices and circuits. Essentially capacitors are comprised by two conductive plates separated by an insulator or dielectric. By using a dielectric material with an electric field dependent dielectric permittivity (dielectric constant), voltage variable capacitors also known as varactors can be realized. The capacitance of a varactor can be tuned by changing the bias voltage across the capacitor. The added flexibility of a voltage tunable capacitance is highly desirable as there is a significant demand in microelectronic and semiconductor device technology to integrate as much functionality as possible into a device, circuit, chip, or die. Varactors have wide applications in electrically tuned devices and circuits such as tunable filters, phase shifters, voltage controlled oscillators, impedance matching networks, and numerous others.
Electric field tunable dielectric materials such as barium strontium titanate (BST), potassium tantalum niobate (KTN), silver tantalum niobate (ATN) and bismuth zinc niobate (BZN) can be integrated into MIM capacitors through thin film deposition and semiconductor processing technology. This allows the integration of many tunable dielectric varactors onto a single wafer and with other circuitry. Most commonly, the MIM capacitor structure is a parallel plate configuration. A metal bottom electrode is deposited on a substrate. A tunable dielectric material is deposited over the bottom electrode. A top electrode is deposited on the tunable dielectric material. Additional removal and patterning steps maybe required to define the MIM structure, but the MIM structure is defined vertically.
Tunable dielectrics such as BST present integration challenges to conventional MIM capacitor fabrication. Tunable dielectrics typically require higher processes temperatures and place limits on the materials that can be employed in device integration. Tunable dielectric processing and process temperature requirements limit electrode metal choices as electrode metal choices limit the temperatures for tunable dielectric processing. This presents two issues: (1) tunable dielectrics cannot be processed at higher temperatures more ideal to produce better dielectric properties such as low dielectric loss, and (2) high conductivity metals such as copper (Cu), silver (Ag), gold (Au), and aluminum (Al) cannot be integrated into the device structure due to process conditions and temperatures that result in metal degradation. There is a need for a fabrication method that can simplify tunable dielectric integration and produce high performance tunable dielectric varactors.